Beam multiplier that can be used as an exit-pupil expander and related system and method

ABSTRACT

A beam multiplier includes a beam-multiplying layer and an adjacent optical layer. The beam-multiplying layer is operable to generate output beamlets of light from an input beam of light, and the optical layer has an adjustable index of refraction. By switching the index of refraction of the optical layer between first and second values, one can switch the beam multiplier to a first state (e.g., “on”) where it generates the output beamlets of light, and can switch the beam multiplier to a second state (e.g., “off”) where it passes the input beam of light but does not generate the output beamlets. And by using the beam multiplier as an exit-pupil expander, one can switch the exit-pupil expander to a first state where it generates multiple exit-pupil images, and to a second state where it generates only a single exit-pupil image.

PRIORITY CLAIM

The present application claims priority from commonly owned U.S. patentapplication Ser. No.10/205,858, filed Jul. 26, 2002 and U.S. patentapplication Ser. No. 10/206,177, filed Jul. 26, 2002, both of whichclaim priority to commonly owned U.S. provisional patent applicationSer. No. 60/350,089, filed Nov. 2, 2001. The present application alsoclaims priority from commonly owned U.S. provisional application Ser.No. 60/565,059, filed Apr. 23, 2004. All of these referencedapplications are incorporated herein by reference.

RELATED APPLICATIONS

This application is related to commonly owned U.S. patent applicationSer. No. ______ (Atty. Docket No.1788-9-9), titled “APPARATUS ANDMETHODS FOR GENERATING MULTIPLE EXIT-PUPIL IMAGES IN AN EXPANDED EXITPUPIL”, which is incorporated by reference and was filed on the same dayas this application.

FIELD OF THE INVENTION

The invention relates generally to image display/projection systems, andmore particularly to an apparatus such as a virtual retinal display thatgenerates an array of exit-pupil images of uniform brightness.

BACKGROUND OF THE INVENTION

A variety of image-display/image-projection devices and techniques areavailable for displaying/projecting graphical or video images—oftencalled video frames—to a viewer. A graphical image, i.e., a graphic,typically changes infrequently or not at all. For example, aflight-instrument graphic of cockpit instruments may overlay a pilot'sview. This graphic may be projected onto a viewing area such as thewindshield, or may be projected directly into the pilot's eyes such thathe/she sees the flight instruments regardless of his/her viewingdirection. Typically, there is little change in this graphic other thanthe movement of the instrument pointers or numbers. Conversely, videoframes are a series of images that typically change frequently to showmovement of an object or the panning of a scene. For example, atelevision displays video frames.

A cathode-ray-tube (CRT) display, such as used in a television orcomputer monitor, is a common image-display/image-projection devicethat, unfortunately, has several limitations. For example, a CRT istypically bulky and consumes a significant amount of power, thus makingit undesirable for many portable or head-mounted applications.

Flat-panel displays, such as liquid-crystal displays (LCDs), organicLEDs, plasma displays, and field-emission displays (FEDs), are typicallyless bulky and consume significantly less power than a CRT having acomparable viewing area. But flat panel displays often lack sufficientluminance and adequate color purity or resolution for many head-mountedapplications.

Referring to FIG. 1, although a scanned-beam display system 71 oftenovercomes the limitations of the above-described displays, the viewermay lose sight of the displayed image if he/she moves his/her eye 73.The display system 71 includes a scanning source 72, which outputs ascanned beam of light that is coupled to a viewer's eye 73 by a beamcombiner 74. In one embodiment, the scanning source 72 includes ascanner (not shown), such as a scanning mirror or acousto-optic scanner,that scans a modulated light beam through a viewer's pupil 75 and onto aviewer's retina 76. In another embodiment, the scanning source 72 mayinclude one or more light emitters (not shown) that are rotated throughan angular sweep. Because such displays scan or project an image throughthe pupil of the viewer's eye, the display's “exit pupil”—defined as anarea, often a plane, in front of the viewer's eye 73 where the image islocated—is limited to the diameter of the viewer's pupil 75, whichtypically ranges from about 2 millimeters (mm) in bright light to about7 mm in dim light. Consequently, the viewer may “lose” the image whenhe/she moves his/her eye 73. A display system similar to the displaysystem 71 is further described in U.S. Pat. No. 5,467,104, which isincorporated by reference.

Referring to FIG. 2, a scanned-beam display system 82 overcomes theproblem of “losing” an image due to eye movement by including adiffraction grating 84 to generate an exit pupil 86, which includes anarray of multiple exit-pupil images 88. Specifically, a modulated lightbeam 92 scans an image 93 onto the diffraction grating 84, where thesize of the image is determined by a scanning angle 2θ. The grating 84diffracts the beam 92 into fractional beams 98 a-98 c, whichrespectively generate exit-pupil images 88 a-88 c as the beam 92 scansthe image 93. Each of the images 88 a-88 c is a replica of, but has alower intensity than, the image 93. An eyepiece 95 collimates the images88 a-88 c to form the exit pupil 86. When the viewer's pupil 75 isaligned with one or more of the images 88 a-88 c, the aligned image orimages 88 converge on an area 100 of the viewer's retina 76 to replicatethe image 93. The intensity of the replicated image is proportional tothe number of images 88 that converge to form the replicated image onthe retinal area 100.

By including multiple exit-pupil images 88, the exit pupil 86effectively increases the viewer's field of view with respect to theimage 93. That is, as long as at least one of the exit-pupil images 88a-88 c is within the viewer's field of view, he/she can see the image93. For example, if the viewer looks down slightly, the exit-pupil image88 b moves out of his/her view, but the image 88 a remains in view andthe image 88 c enters his/her view. Therefore, even though the viewerhas moved his/her eye 73, he/she still views the image 93 via theexit-pupil images 88 a and 88 c. A scanned-beam display system that issimilar to the display system 82 is further described in U.S. Pat. No.5,701,132, which is incorporated by reference.

Unfortunately, the exit-pupil images 88 generated by the scanned-beamdisplay 82 often have non-uniform intensities, which may annoy ordistract the viewer. Specifically, the diffraction grating 84 istypically designed for a single wavelength of light, but the image beam92 typically includes other wavelengths in addition to this singlewavelength. These other wavelengths often cause the exit-pupil images 88to have different intensities. Therefore, one typically limits theintensity of the beam 92 so that the brighter exit-pupil images 88 arenot too bright for the viewer. But this may cause some of the dimmerimages 88 to be too dim for the viewer to see, thus causing “holes” inthe exit pupil 86. Furthermore, even if none of the images 88 are toodim for the viewer to see, the differences in intensity among the images88 may annoy or distract the viewer as he/she shifts his/her gaze.

SUMMARY OF THE INVENTION

In one embodiment of the invention, a beam multiplier includes abeam-multiplying layer and an adjacent optical layer. Thebeam-multiplying layer is operable to generate output beamlets of lightfrom an input beam of light, and the optical layer has an adjustableindex of refraction.

By switching the index of refraction of the optical layer between firstand second values, one can switch the beam multiplier to a first state(e.g., “on”) where it generates the output beamlets of light, and canswitch the beam multiplier to a second state (e.g., “off”) where itpasses the input beam of light but does not generate the outputbeamlets. And by using the beam multiplier as an exit-pupil expander,one can switch the exit-pupil expander to a first state where itgenerates multiple exit-pupil images, and to a second state where itgenerates only a single exit-pupil image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a conventional scanned-beam display system.

FIG. 2 is a diagram of a conventional scanned-beam display system thatincludes an exit-pupil expander.

FIG. 3 is a diagram of a scanned-beam display system according to anembodiment of the invention.

FIG. 4 is a diagram of the lens assembly of FIG. 3 according to anembodiment of the invention.

FIG. 5A is a diagram of the exit-pupil images produced by thediffraction grating of FIG. 4.

FIG. 5B is a diagram showing the relative unfiltered intensities of theexit-pupil images of FIG. 5A.

FIG. 5C is a diagram showing the relative filtered intensities of theexit-pupil images of FIGS. 5A and 5B.

FIG. 6 is a diagram of the lens assembly of FIG. 3 according to anotherembodiment of the invention.

FIG. 7A is a diagram of an exit-pupil array produced by the diffractiongrating of FIG. 6.

FIG. 7B is a diagram showing the relative unfiltered intensities of theexit-pupil images of FIG. 7A.

FIG. 7C is a diagram showing the relative filtered intensities of theexit-pupil images of FIGS. 7A and 7B.

FIG. 8 is the lens assembly of FIG. 3 according to yet anotherembodiment of the invention.

FIG. 9 is a cross-sectional view of the diffraction gratings of FIGS. 4and 6 according to an embodiment of the invention.

FIG. 10 is a cross-sectional view of the diffraction gratings of FIGS. 4and 6 according to another embodiment of the invention.

FIG. 11 is a cross-sectional view of the diffraction grating of FIG. 8according to an embodiment of the invention.

FIG. 12 is a cross-sectional view of the diffraction grating of FIG. 8according to another embodiment of the invention.

FIG. 13A is a close-up plan view of the diffraction grating of FIG. 4according to an embodiment of the invention.

FIG. 13B is a close-up plan view of one of the quadrants of thediffraction grating of FIG. 13A.

FIG. 14 is a close-up plan view of the diffraction grating of FIG. 6according to an embodiment of the invention.

FIG. 15 is a cross-sectional view of an on/off diffraction grating thatcan be used in the lens assemblies of FIGS. 4, 6, and 8 according to anembodiment of the invention.

FIG. 16 is a cross-sectional view of an on/off diffraction grating thatcan be used in the lens assemblies of FIGS. 4, 6, and 8 according toanother embodiment of the invention.

FIG. 17 is a cross-sectional view of a refractive beam multiplier thatcan be used as an exit-pupil expander according to an embodiment of theinvention.

FIG. 18 is a cross-sectional view of a refractive beam multiplier thatcan be used as an exit-pupil expander according to another embodiment ofthe invention.

DESCRIPTION OF THE INVENTION

FIG. 3 is a diagram of a scanned-beam display system 100 that generatesan exit pupil 102 having exit-pupil images (not shown in FIG. 3) ofuniform or approximately uniform intensity according to an embodiment ofthe invention. The display system 100 includes an image-beam source 104for generating an image beam 106, a scanning assembly 108 for scanningthe beam 106, and a lens assembly 110. The assembly 110 includes adiffraction grating 112 for generating exit-pupil images havingdifferent intensities, and includes an ocular 114 for filtering theexit-pupil images from the grating 112 to generate the exit pupil 102.The lens assembly 110 is further discussed below in conjunction withFIGS. 4-14.

In operation of the display system 100, the source 104 modulates thebeam 106 to generate pixels of a scanned image (not shown in FIG. 3),and the scanning assembly 108 scans the modulated beam 106 onto thediffraction grating 112. Although multiple paths of the scanned beam 106are shown to illustrate the generation of the exit pupil 102, it isunderstood that the beam travels along only one path at a time. Thegrating 112 diffracts the beam 106, and thus generates an array ofexit-pupil images (not shown in FIG. 3) having a center image that isbrighter than the visible peripheral images, which have the same orapproximately the same intensity. The ocular 114 collects this array ofexit-pupil images, and, at an intermediate exit-pupil plane (not shownin FIG. 3) within the ocular, partially attenuates the center exit-pupilimage or fully attenuates, i.e., blocks, the center exit-pupil image. Bypartially or fully attenuating the center exit-pupil image, the ocular114 generates each of the exit-pupil images of the exit pupil 102 havingthe same or approximately the same intensity as the other exit-pupilimages. A beam source such as the source 104 and a scanning assemblysuch as the assembly 108 are discussed in commonly owned U.S. patentapplication Ser. No. 09/369,676, titled SCANNED DISPLAY WITH SWITCHEDFEEDS AND DISTORTION CORRECTION, which is incorporated by reference.

Still referring to FIG. 3, although the lens assembly 110 is describedas generating the exit pupil 102 from a scanned beam 106, the assembly110 can also generate the exit pupil 102 from an image that is projectedonto the diffracting grating 112. For example, the image-beam source 104and the scanning assembly 108 may be replaced with a planar imagedisplay (not shown) such as a light-emitting-diode (LED) matrix display,a liquid-crystal (LC) matrix display, or a cathode-ray-tube (CRT)display. Such planar displays and the optical components (not shown)that would be disposed between such a display and the grating 112 areknown; consequently, discussion of such displays and components isomitted for brevity.

Furthermore, although described as producing a center image that isbrighter than each of the uniformly bright peripheral images, thediffracting grating 112 may generate an array of exit-pupil imageshaving a different intensity pattern. With such a grating 112, theocular 114 can filter exit-pupil images other than the center image.

FIG. 4 is a diagram of the lens assembly 110 of FIG. 3 according to anembodiment of the invention where the viewer's eye (FIG. 3) is on thesame side of the assembly 110 as the scanning assembly 108 (FIG. 3). Thediffracting grating 112 generates an array of exit-pupil images in anintermediate exit-pupil plane 120, where a center exit-pupil image 122has a first intensity and the peripheral exit-pupil images 124 each haveor approximately have a second intensity that is less than the firstintensity. The ocular 114 includes an obscuration plate 126, whichblocks the center exit-pupil image 122 such that the peripheralexit-pupil images 124 of the exit pupil 102 each have the same orapproximately the same intensity.

FIG. 5A shows the array of exit-pupil images 122 and 124 in the plane120 of FIG. 4, and FIG. 5B shows the relative intensities of the imagesof FIG. 5A at a non-design wavelength of the beam 106 according to anembodiment of the invention.

Referring to FIGS. 4, 5A, and 5B, the diffraction grating 112 (discussedin greater detail below in conjunction with FIGS. 9, 10, 13A, and 13B)is a binary-phase, even-orders-missing (EOM) diffraction gratingdesigned for a single wavelength of light. At the design wavelength, thecenter exit-pupil image 122 has zero intensity, i.e., is dark, and theperipheral exit-pupil images 124 have the same or approximately the samenon-zero intensity. But, as shown in FIG. 5B, as the wavelengthincreases or decreases from the design wavelength, the center image 122gets brighter while the other images 124 get dimmer—although each image124 gets dimmer, it maintains the same or approximately the sameintensity as the other images 124 for any wavelength. Consequently, inone embodiment, one designs the EOM grating 112 for the median of theexpected wavelengths in the image beam 106 to maximize the minimumintensity of the peripheral images 124. Therefore, because the beam 106includes visible wavelengths of light that enable it to scan an imagethat is visible to the human eye, in one embodiment the EOM grating 112is designed for approximately 545 nanometers (nm)—the wavelength ofyellow/green light—because this is the approximate median wavelength ofvisible light, which ranges from 420 nm (violet light) to 670 nm (redlight).

More specifically, referring to FIGS. 5A and 5B, the exit-pupil image122 is the 0^(th)-order image of the array in the intermediateexit-pupil plane 120, the images 124 respectively represent theodd-order (1^(st), 3^(rd), 5^(th), 7th, . . . ) images, and the images130 represent the missing—and thus the dark—even-order (2^(nd), 4^(th),6^(th), 8th, . . . ) images. Although the images 122, 124, and 130 arecircular in FIG. 5A, they may be square as in FIG. 5B or may have othershapes. As discussed above, the center image 122 is dark when the beam106 (FIG. 4) includes the design wavelength and only the designwavelength of the grating 112 (FIG. 4), and has a nonzero intensity whenthe beam 106 includes a wavelength or wavelengths other than the designwavelength. Conversely, the even-order images 130 are dark for anywavelength or combination of wavelengths. Furthermore, as shown in FIG.5B, the center image 122 may be significantly brighter than theperipheral images 124 for a wavelength or a combination of wavelengthsother than the design wavelength. Therefore, if a viewer (not shown inFIGS. 5A-5B) could look at the array in the intermediate exit-pupilplane 120, the image he sees would appear significantly brighter whenhis pupil is directed toward the center of the array than it would whenhis pupil is directed toward the periphery of the array. As discussedabove, this change in brightness as the viewer shifts his/her gaze mayannoy or distract him/her.

FIG. 5C shows the relative intensities of the images 122 and 124 of FIG.5A in the exit pupil 102 (FIG. 4) after the obscuration plate 126 (FIG.4) filters them. As discussed below, the plate 126 completely blocks thecenter image 122, which is thus dark and not shown in the exit pupil102. Furthermore, the images 124 each have the same or approximately thesame intensity. In one embodiment, the images 124 are considered to havethe same or approximately the same intensity if the difference inintensity between the brightest and dimmest images 124 divided by thesum of the intensities of the brightest and dimmest images 124 equals0.30 or less.

More specifically, referring again to FIG. 4, the obscuration plate 126is light transmissive to all the peripheral exit-pupil images 124, butis opaque to the center exit-pupil image 122, which the plate 126 thusblocks from propagating to the exit pupil 102. The plate 126 is locatedat the intermediate exit-pupil plane 120, and an appropriately sized andlocated blocking element 132 is formed as an integral part of or isattached to the plate 126. One can make the plate 126 from any lighttransmissive material and design it so that the plate 126 imparts littleor no diffraction to the exit-pupil images 124 passing through it.Furthermore, one can use conventional techniques to make the appropriatesection of the plate 126 opaque to form the blocking element 132 as anintegral part of the plate 126. Or, one can attach opaque material tothe plate 126 to form the blocking element 132 using any type ofadhesive, preferably adhesive that is not adversely affected by heat,light, or moisture.

Although the plate 126 is located at the intermediate exit-pupil plane120, the lens assembly 110 may include optical relays (not shown) toproduce additional intermediate or exit-pupil planes where the plate 126may be located.

Still referring to FIG. 4, the ocular 114 also includes lenses 134, amirror 136, and a partially transmissive mirror 138. In one embodiment,the lenses 134 are cylindrical graded index (GRIN) lenses formed tominimize any additional diffraction of the exit-pupil images 122 and 124passing through them. The lenses 134 gather the exit-pupil images 122and 124 emanating from the EOM grating 112 and focus them at theintermediate exit-pupil plane 120, where the obscuration plate 126 islocated. The mirror 136 reflects the exit-pupil images 124 (the plate120 blocks the center image 122) onto the partially transmissive mirror138, which reflects these images onto a mirror 139. The mirror 139redirects the exit-pupil images 124 back through the partiallytransmissive mirror 138 to a display exit-pupil plane 140, where theyform the exit pupil 102.

By using the partially transmissive mirror 138, the display system 100(FIG. 3) may be advantageously mounted at any location not along theviewer's line of sight. For example, the scanning assembly 108 (FIG. 3),the EOM grating 112, the lenses 134 and the obscuration plate 126 can bemounted on a helmet or hat that, when worn, locates these componentsbehind or adjacent the viewer's eyes. By removing these components fromthe viewer's line of sight, the viewer can simultaneously view theexit-pupil images 124 and his/her surroundings. For fighter pilots, racecar drivers, or anyone who needs to maintain visual contact with thesurrounding environment while also maintaining visual contact withengine gauges, gun sights or the like, this is significant.

Although FIG. 4 illustrates an ocular 114 having a specific number oflenses 134, mirrors 136 and partially transmissive mirror 138 in aspecific combination, any number and combination may be used to gatherthe exit-pupil images 122 and 124 and focus them at intermediate anddisplay exit-pupil planes 120 and 140. Also, even though FIG. 4 shows anEOM grating 112, the lens assembly 110 can include other types ofdiffraction gratings.

Referring again to FIGS. 4, 5A, 5B, and 5C, excessive intensity in thecenter exit-pupil image 122 can be created by a variety of means otherthan the image beam 106 having a wavelength different than the designwavelength of the EOM grating 112. For instance, minor manufacturingdefects in the EOM grating 112 or the attachment of a liquid or fineparticulate dust to the EOM grating 112 can generate excessiveintensities in the central exit-pupil image 122. In addition, a failureof the scanning assembly 108 (FIG. 3) or the light source 104 (FIG. 3)can generate excessive intensities in the central exit-pupil image 122.Excessive intensity in the image 122 also may be created intentionallyto generate a viewable exit pupil 102 in bright environments. But byblocking the image 122 in the intermediate plane 120, the plate 126allows the images 124 in the exit-pupil 102 to have uniform orapproximately uniform intensities regardless of why the image 122 isbrighter than the images 124.

In addition, in another embodiment, the EOM grating 112, the obscurationplate 126, or both can be switched “on” or “off.” When the grating 112is “on”, it generates the exit-pupil images 122, 124, and 130 asdiscussed above; when it is “off”, it acts as non-diffracting glass suchthat the lens assembly 110 generates only the 0^(th)-order exit-pupilimage 122 in the intermediate exit-pupil plane 120. Similarly, when theobscuration plate 126 is “on”, it attenuates the exit-pupil image 122 asdiscussed above; when it is “off”, it allows the exit-pupil image 122 topass through just as it allows the odd-order images 124 to pass through.Therefore, when both the grating 112 and plate 126 are “on”, the lensassembly 110 generates the exit pupil 102 as discussed above.Conversely, when both the grating 112 and plate 126 are “off”, the lensassembly 110 generates the exit-pupil image 122 and only the exit-pupilimage 122 in the exit pupil 102. One application of turning both thegrating 112 and the plate 126 “off” is where a viewer's eye (FIG. 3) isstable and focused only on the center of the exit pupil 102. Therefore,the peripheral exit-pupil images 124 are not needed, and they can beturned “off” so that all the power goes to the center exit-pupil image122. This makes the image 122 brighter, and may allow one to reduce theimage power. Such an on/off obscuration plate 126 can include aconventional LC material (not shown) or a conventional mechanicalshutter (not shown) for the blocking element 132. Examples of an on/offgrating 112 are discussed below in conjunction with FIGS. 15 and 16.

Moreover, in another embodiment, the obscuration plate 126 attenuatesexit-pupil images other than the exit-pupil image 122 to generate theexit-pupil images 124 having non-uniform intensities. For example, apilot may prefer the outer images 124 to be dimmer than the inner images124. Such an obscuration plate 126 can include a conventional blockingor filtering material in the appropriate regions to cause the desireddimming or blocking. Or, the plate 126 may include LC material (notshown) or conventional mechanical shutter(s)/filters in these regions toallow turning of the dimming/blocking function “on” or “off”.

Furthermore, although a finite number of odd-order and even-orderexit-pupil images 124 and 130 are shown in FIGS. 5A-5C, it is understoodthat there may be an infinite number of these images, where thebrightness of an image 124 is inversely proportional to its distancefrom the center image 122, particularly outside of a predeterminedradius from the center image. In addition, although the even-orderimages 130 are disclosed as having zero intensity, imperfections in theEOM grating or other components of the lens assembly 110 may cause someof the even-order images to have nonzero intensities that are inverselyproportional to distance from the center image 122. However, because theintensities of the even-order images 130 are typically much less thanthe intensities of the odd-order images 124, the even-order intensitiesare referred to as having a zero intensity to the human eye.

FIG. 6 is a diagram of the lens assembly 110 of FIG. 3 according toanother embodiment of the invention. The diffracting grating 112 is abinary-phased diffraction grating that generates an array of exit-pupilimages in an intermediate exit-pupil plane 120 where the centerexit-pupil image 122 has a first intensity and the remaining exit-pupilimages 124 each have or approximately have a second intensity that isless than the first intensity. The ocular 114 includes a filter 142,which unlike the obscuration plate 126 of FIG. 4, partially attenuatesthe center exit-pupil image 122 such that all the exit-pupil images 122and 124 of the exit pupil 102 have the same or approximately the sameintensity.

FIG. 7A shows the array of exit-pupil images 122 and 124 in theintermediate plane 120 of FIG. 6, and FIG. 7B shows the relativeintensities of the images of FIG. 7A at a non-design wavelength of thebeam 106 according to an embodiment of the invention.

Referring again to FIG. 6, the lens assembly 110 is similar to the lensassembly 110 of FIG. 4 except for two major differences. A binary-phase,(simple) diffraction grating 112 replaces the EOM grating of FIG. 4, andthe filter 142 replaces the obscuration plate of FIG. 4. Therefore, thefollowing discussion focuses on the simple grating 112 and the filter142 and refers to the remaining, previously discussed structure of theocular 114 using the previously identified names and numbers.

Referring to FIGS. 6, 7A, and 7B, the simple grating 112 (discussed ingreater detail below in conjunction with FIGS. 9, 10, and 14) isdesigned for a single wavelength of light. At this wavelength, thecenter exit-pupil image 122 and the peripheral exit-pupil images 124have the same or approximately the same non-zero intensities. But, asshown in FIG. 7B, as the wavelength increases or decreases from thissingle wavelength, the center image 122 gets brighter while theperipheral images 124 get dimmer—although each image 124 gets dimmer, itmaintains the same or approximately the same intensity as the otherimages 124 for any wavelength. Consequently, as discussed above inconjunction with FIGS. 4-5C, in one embodiment one designs the simplegrating 112 for 545 nm—the approximate median wavelength of visiblelight—to maximize the minimum intensity of the peripheral images 124 andto minimize the maximum intensity of the center image 122. Furthermore,as discussed above in conjunction with FIGS. 4-5C, manufacturing defectsin or dust/liquid on the grating 112 may cause the center image 122 tobe brighter than the peripheral images 124.

More specifically, referring to FIGS. 7A and 7B, the exit-pupil image122 is the 0^(th)-order image of the array in the intermediateexit-pupil plane 120, and the images 124 respectively represent the evenand odd order (1^(st), 2^(nd), 3^(rd), 4^(th), 5^(th), 6^(th), 7^(th) .. . ) images. Although the images 122 and 124 are circular in FIG. 7A,they may be square as in FIG. 7B or may have other shapes. As discussedabove, the center image 122 has the same or approximately the sameintensity as the peripheral images 124 when the beam 106 (FIG. 6)includes the design wavelength and only the design wavelength of thesimple grating 112 (FIG. 6). And, as shown in FIG. 7B, the center image122 has a greater intensity than the peripheral images 124 when the beam106 includes a wavelength or wavelengths other than the designwavelength. Therefore, if a viewer (not shown in FIGS. 7A-7B) could lookat the array in the intermediate exit-pupil plane 120, the image he seeswould appear significantly brighter when his pupil is directed towardthe center of the array than it would when his pupil is directed towardthe periphery of the array. As discussed above, this change inbrightness as the viewer shifts his/her gaze may annoy or distracthim/her.

FIG. 7C shows the relative intensities of the images 122 and 124 of FIG.7A in the exit-pupil plane 102 after they are filtered by the filter 142(FIG. 6). As discussed below, the filter 142 partially attenuates thecenter image 122, thus causing it to have the same or approximately thesame intensity as each of the peripheral images 124 in the plane 102. Inone embodiment, the images 122 and 124 are considered to have the sameor approximately the same intensity if the difference in intensitybetween the brightest and dimmest images 122 and 124 divided by the sumof the intensity of the brightest and dimmest images 122 and 124 equals0.30 or less.

More specifically, referring to FIG. 6, the filter 142 is equally lighttransmissive to all the exit-pupil images 124, but is less transmissiveto the center exit-pupil image 122, which the filter 142 thus attenuatesbut does not block. The filter 142 is located at the intermediateexit-pupil plane 120, and an appropriately sized and located filteringelement 144 is formed as an integral part of or is attached to thefilter 142. One can make the filter 142 from any light transmissivematerial and design it so that the filter 142 imparts little or nodiffraction to the exit-pupil images 122 and 124 passing through it.Furthermore, one can use conventional techniques to make the appropriatesection of the filter 142 partially transmissive to form the filteringelement 144 as an integral part of the filter 142. Or, one can attachpartially transmissive material to the filter 142 to form the filteringelement 144 using any type of adhesive, preferably adhesive that is notadversely affected by heat, light, or moisture.

Although the filter 142 is located at the intermediate exit-pupil plane120, the lens assembly 110 may include optical relays (not shown) toproduce additional intermediate or exit-pupil planes where the filter142 may be located. Moreover, in some applications, it may be desirableto place filters at more than one intermediate or exit-pupil plane.

In addition, although FIG. 6 illustrates an ocular 114 having a specificnumber of lenses 134, mirrors 136 and partially transmissive mirrors 138in a specific combination, any number and combination may be used togather the exit-pupil images 122 and 124 and focus them at intermediateand display exit-pupil planes 120 and 140. Also, even though FIG. 6shows a simple grating 112, the lens assembly 110 can include othertypes of diffraction gratings.

Furthermore, the simple grating 112, the filter 142, or both can bedesigned to switch “on” or “off” as discussed above in conjunction withFIGS. 4-5C.

In addition, although a finite number of odd-order and even-orderexit-pupil images 124 are shown in FIGS. 7A-7C, it is understood thatthere may be an infinite number of these images, where the brightness ofan image 124 is inversely proportional to its distance from the centerimage 122, particularly outside of a predetermined radius from thecenter image 122.

FIG. 8 is a diagram of the lens assembly 110 of FIG. 3 according toanother embodiment of the invention. This lens assembly 110 is similarto the previously described lens assembly of FIG. 4 except for one majordifference; a multi-phase, even-orders-missing (multi-phase EOM)diffraction grating 112 replaces the binary-phase, EOM grating 112 ofFIG. 4. Typically, the multi-phase EOM grating 112 (discussed in greaterdetail below in conjunction with FIGS. 11 and 12) is less sensitive toan increase or decrease from the design wavelength than the binary-phaseEOM grating of FIG. 4. That is, for a given increase or decrease in thedesign wavelength, the intensities of the center exit-pupil image 122and the peripheral exit-pupil images 124 change less with the multiphaseEOM grating 112 than they do with the binary-phase EOM grating.Otherwise, the structure and operation of the lens assembly 110 of FIG.4 is similar to the structure and operation of the lens assembly 110 ofFIG. 8.

FIG. 9 is a cross-sectional view of the binary-phase EOM diffractiongrating 112 of FIG. 4 and the simple binary-phase diffraction grating112 of FIG. 6 according to an embodiment of the invention. The grating112 has a beam-incident side 146 and a beam-emanating side 148, whichincludes raised and recessed surfaces 150 and 152. For an incident lightbeam 106 having the design wavelength of the grating 112, components(not shown) of the light beam 106 emanating from the surface 150 are orare approximately π radians (180°) out of phase with components (notshown) emanating from the surface 152. Because the side 148 has only twosurfaces 150 and 152, the emanating light-beam components can have onlyone of two phases; hence the term “binary phase.” By shifting the phaseof the beam components emanating from the surface 150 with respect tothe beam components emanating from the surface 152, the grating 112generates an interference pattern that results in the array ofexit-pupil images 122 and 124 of FIGS. 5A-5B or of FIGS. 7A-7B. Thearrows show the relative directions of the resulting interferencecomponents, and the arrow labels indicate the orders of the interferencecomponents, which correspond to the orders of the exit-pupil images 122and 124 of FIGS. 5A and 7A. Although the EOM diffraction grating 112 ofFIG. 4 and the simple grating 112 of FIG. 6 may have similar crosssections, they have different patterns of the surfaces 150 and 152formed on the side 148 as discussed below in conjunction with FIGS.13A-14.

Still referring to FIG. 9, in one embodiment the beam-incident side 146and the raised and recessed surfaces 150 and 152 of the grating 112 areplanar or approximately planar and are parallel or approximatelyparallel to each other. To produce opposite phases in the emanatingcomponents (not shown) of the incident light beam 106, the difference bin height between the surfaces 150 and 152 is determined from thefollowing equation, which is discussed further in “Diffractive ExitPupil Expander for Display Applications,” by Hakan Urey, to be publishedin a 2001 feature issue of Applied Optics focusing on Diffractive Opticsand Micro-optics, and which is incorporated by reference:$\begin{matrix}{b = \frac{\lambda\phi}{2{\pi\left( {n - 1} \right)}}} & (1)\end{matrix}$where λ equals the design wavelength of the grating 112 in the medium(e.g., air) in which the grating 112 is disposed, φ is the desired phasedifference (here π radians) between the beam components emanating fromthe surface 150 and those emanating from the surface 152, and n is theindex of refraction for the material from which the grating 112 isconstructed. As discussed above in FIGS. 4 and 6, because the incidentlight beam 106 often includes wavelengths other than λ, some wavelengthsof the emanating beam components may not be π radians out of phase withone another. This typically causes the center exit-pupil image 122 to bebrighter than the peripheral images 124, and thus typically requiresthat the ocular 114 include the block 126 or filter 142 as discussedabove in conjunction with FIGS. 4-7C.

In operation of the diffracting grating 112, at any particular instantthe scanning assembly 108 (FIG. 3) directs the incident light beam 106onto a region 154 of the side 146, the region 154 having a dimension d.The beam 106 propagates through the grating 112 at a refraction angle θ₂that depends on the index of refraction n of the grating and the angleof incidence θ₁. As discussed above, the phase differences caused by thesurfaces 150 and 152 generate an interference pattern that isgraphically represented by the resulting interference components. Theangles α at which these resulting interference components emanate fromthe side 148 depend on the angles θ₁ and θ₂ and the wavelength of theincident light beam 106. Each of these resulting interference componentsrespectively forms a pixel of the corresponding exit-pupil image 122 and124 of FIGS. 5A or 7A. As the scanning assembly 108 (FIG. 3) scans thebeam 106 across the side 146, the interference pattern generatesadditional pixels as the resulting components effectively scan therespective exit-pupil images 122 and 124.

In one embodiment, the grating 112 can be made of any light-transmissivematerial, such as glass, plastic, or the like, having an index ofrefraction different from the medium surrounding it. Furthermore, onecan conventionally etch the surfaces 150 and 152 on the side 148.

Still referring to FIG. 9, although the side 146 and surfaces 150 and152 are described as being planar and parallel, in other embodimentsthey may have different characteristics. For example, the side 146 andthe surfaces 150 and 152 may be convex or concave. In addition, althoughthe grating 112 is described and shown with the beam-emanating side 148including the raised and recessed surfaces 150 and 152, thebeam-incident site 146 may include the surfaces 150 and 152 instead ofthe beam-emanating side 148.

FIG. 10 is a cross-sectional view of the binary-phase EOM diffractiongrating 112 of FIG. 4 and the simple binary-phase diffraction grating112 of FIG. 6 according to another embodiment of the invention. Thebinary-phase diffraction grating 112 of FIG. 10 is similar to thegrating 112 of FIG. 9 except that the grating 112 of FIG. 10 has abeam-reflecting side 156, which enables one to locate the ocular 114 onthe same side of the grating as the scanning assembly 108 (FIG. 3).

More specifically, the grating 112 of FIG. 10 has a beam-incident side146 and a beam-reflecting side 156, which includes raised and recessedsurfaces 150 and 152 that each have a reflective coating 158. For anincident light beam 106 having the design wavelength of the grating 112,components (not shown) of the light beam 106 reflected from the surface150 are or are approximately π radians (180°) out of phase withcomponents (not shown) reflected from the surface 152. Thus, aspreviously discussed in conjunction with FIG. 9, the grating 112generates an interference pattern that results in the array ofexit-pupil images 122 and 124 of FIGS. 5A-5B or of FIGS. 7A-7B. Thearrows show the relative directions of the resulting interferencecomponents, and the arrow labels indicate the component orders, whichcorrespond to the orders of the exit-pupil images 122 and 124 of FIGS.5A and 7A. Although the binary-phase EOM grating 112 of FIG. 4 and thesimple binary-phase grating 112 of FIG. 6 may have similar crosssections, they have different patterns of the surfaces 150 and 152formed on the side 156 as discussed below in conjunction with FIGS.13A-14.

Still referring to FIG. 10, to produce opposite phases in the reflectedbeam components (not shown) of the incident light beam 106, thedifference c in height between the surfaces 150 and 152 is determinedfrom the following equation: $\begin{matrix}{c = \frac{\lambda\phi}{2{\pi(n)}}} & (2)\end{matrix}$where, λ, φ, and n represent the same quantities that they do inequation (1).

That is, referring to equation (1) c≠b because the beam-reflecting side156 reflects the incident light beam 106 back toward the beam-incidentside 146, and thus the incident beam 106 travels through an additionaldistance of the grating 112. Therefore, the difference c in heightbetween the surfaces 150 and 152 accounts for this additional travel.

In operation of the diffraction grating 112 of FIG. 10, at anyparticular instant the scanning assembly 108 (FIG. 3) directs theincident light beam 106 onto a region 154 of the side 146, the region154 having a dimension d. The beam 106 propagates through the grating112 at a refraction angle θ₂ that depends on the index of refraction nof the grating and the angle of incidence θ₁. The surfaces 150 and 152reflect the beam 106 back toward the beam-incident side 146 and generatethe interference pattern that is graphically represented by theresulting interference components. As previously discussed inconjunction with FIG. 9, each of these resulting components respectivelyforms a pixel of the corresponding exit-pupil image 122 and 124 of FIGS.5A or 7A.

In one embodiment, the grating 112 can be made of any light-transmissivematerial, such as glass, plastic, or the like, having an index ofrefraction different from the medium surrounding it. Furthermore, thereflective coating 158 can be any conventional coating that reflectslight. Moreover, one can conventionally etch the surfaces 150 and 152 onthe side 158 and attach the reflective coating 158 using conventionaltechniques.

Still referring to FIG. 10, although the side 146 and surfaces 150 and152 are described as being planar and parallel, in other embodimentsthey may have different characteristics. For example, the side 146 andthe surfaces 150 and 152 may be convex or concave. Furthermore, althoughthe beam-reflecting side 156 is described as having a reflective coating158 on the surfaces 150 and 152, the reflective coating may be omittedand a reflector (not shown) that is separate from the grating 112 andspaced a distance from the beam-reflecting side 156 may be included toreflect the incoming light beam 106. To accommodate such an externalreflector, one can adjust the height difference c and the pattern of thesurfaces 150 and 152 as needed so that the grating 112 diffracts thelight beam 106 as desired. Such a combination of grating and externalreflector may form, for example, a reflective exit-pupil expander.

FIG. 11 is a cross-sectional view of the multi-phase EOM diffractiongrating 112 of FIG. 8 according to an embodiment of the invention. Themulti-phase EOM grating is similar to the binary-phase EOM grating 112of FIG. 9 except that the multi-phase EOM grating 112 has three or moresurfaces—here three surfaces 160, 162 and 164—on the beam-emanating side166 instead of only two surfaces (150 and 152 in FIG. 9).

Still referring to FIG. 11, the grating 112 has a beam-incident side 146and the beam-emanating side 166, which includes raised and recessedsurfaces 160, 162, and 164. For an incident light beam 106 having thedesign wavelength of the grating 112, components (not shown) of thelight beam 106 emanating from the surface 160 are or are approximately2π/3 radians (120°) out of phase with beam components (not shown)emanating from the surfaces 162 and 164. And, the components (not shown)of the light beam 106 emanating from the surface 162 are or areapproximately 2π/3 radians (120°) out of phase with beam components (notshown) emanating from the surfaces 160 and 164. But although equalheight differences between the surfaces 160, 162, and 164 and equalphase differences between the beam components emanating therefrom arediscussed, these height and phase differences may be unequal. Becausethe side 166 has more than two surfaces 160, 162, and 164, the emanatinglight-beam components have more than two phases; hence the term“multiple phase.” By shifting the phase of the beam components emanatingfrom the surface 160 with respect to the beam components emanating fromthe surfaces 162 and 164, the grating 112 generates an interferencepattern that results in the array of exit-pupil images 122 and 124 ofFIGS. 5A-5B. The arrows show the relative directions of the resultinginterference components, and the arrow labels indicate theinterference-component orders, which correspond to the orders of theexit-pupil images 122 and 124 of FIGS. 5A-5B.

Still referring to FIG. 11 in one embodiment, the beam-incident side 146and the raised and recessed surfaces 160, 162 and 164 are planar orapproximately planar and are parallel or approximately parallel to eachother. To produce a 2π/3 radians (120°) difference in phase between theemanating components (not shown) of the incident light beam 106, thedifference e in height between the surfaces 162 and 164 and thedifference f in height between the surfaces 160 and 162 are given by thefollowing equations: $\begin{matrix}{e = \frac{\lambda\phi}{3{\pi\left( {n - 1} \right)}}} & (3) \\{f = \frac{\lambda\phi}{3{\pi\left( {n - 1} \right)}}} & (4)\end{matrix}$where λ, φ, and n represent the same quantities that they do inequations (1) and (2).

As discussed above in FIG. 8, because the incident light beam 106 oftenincludes wavelengths other than λ, some wavelengths of the emanatingbeam components may not be 2π/3 radians out of phase with one another.This may cause the center exit-pupil image 122 to be brighter than theperipheral images 124, and thus may require that the ocular 114 includethe block 126 or a partial attenuator.

In operation, the diffracting grating 112 is similar to the diffractiongrating 112 of FIG. 9 except that three surfaces 160, 162 and 164 causethe phase differences, and thus generate the interference pattern, asthe beam 106 exits the side 166. As the scanning assembly 108 (FIG. 3)scans the beam 106 across the side 146, the interference patterngenerates the pixels of the exit-pupil images 122 and 124 of FIG. 5A.

In one embodiment, the grating 112 can be made of any light-transmissivematerial, such as glass, plastic, or the like, having an index ofrefraction different from the medium surrounding it. Furthermore, onecan conventionally etch the surfaces 160, 162, and 164 on the side 166.

Still referring to FIG. 11, although the side 146 and surfaces 160, 162and 164 are described as being planar and parallel, in other embodimentsthey may have different characteristics. For example, the side 146 andthe surfaces 160, 162 and 164 may be convex or concave. Also, thegrating 112 can have more than the three surfaces 160, 162 and 164 onthe beam-emanating side 166 to cause phase differences among theemanating components of the beam 106. In addition, although thediffraction grating 112 is a multi-phase EOM grating, the grating 112also can be a simple multi-phase grating with a cross section similar tothe multi-phase EOM grating. Such a simple multi-phase grating can beused in the lens assembly 110 of FIG. 6. However, even though the crosssections are similar, the multi-phase EOM and simple multi-phasegratings would have different patterns of the surfaces 160, 162 and 164formed on the side 166 as discussed below in conjunction with FIGS.13A-14.

FIG. 12 is a cross-sectional view of the multi-phase diffraction grating112 of FIG. 8 according to another embodiment of the invention. Thegrating 112 is a multi-phase grayscale (grayscale) diffraction gratingand is similar to the multi-phase EOM grating 112 of FIG. 11 with onemajor exception. The beam-emanating side 168 includes a surface 170defined by a continuous curve instead of three discrete surfaces 160,162, and 164 (FIG. 11).

Referring to FIGS. 13A-14, the construction of the diffraction gratings112 of FIGS. 4, 6, and 9 are discussed.

FIGS. 13A and 13B are plan views of the interference-pattern-emanatingside 148 of the binary-phase EOM diffracting grating 112 of FIGS. 4 and9 according to an embodiment of the invention. The light and darkportions of the grating 112 represent a relief pattern. In oneembodiment, the light portions of the grating 112 represent the raisedsurfaces 150 (FIG. 9), and the dark portions represent the recessedsurfaces 152; but in another embodiment, the dark portions represent theraised surfaces and the light portions represent the recessed surfaces.But either way, the grating 112 generates the exit-pupil images 122 and124 of FIG. 5A.

Referring to FIG. 13A, each quadrant, i.e., cell region, 172 has thesame relief pattern as the other regions 172, has dimensions d×d, andrepresents a respective pixel region of the grating 112. That is, whenthe beam 106 (FIG. 9) strikes a region 172, the grating 112 generatesthe corresponding pixels of the exit-pupil images 122 and 124 (FIG. 5A).Therefore, although omitted for clarity, the grating 112 typicallyincludes an array of regions 172, the array having the same dimensions,in pixels, as the scanned and exit-pupil images. In one embodiment, thecross section of the beam 106 (FIG. 4) where it strikes the grating 112is square and has or approximately has the dimensions d×d, although thecross section may be circular and have or approximately have a diameterof d. The beam cross section being approximately the same size as eachregion 172 typically ensures that the peripheral exit-pupil images 124(FIG. 5A) each have same or approximately the same intensity, and thatthe images 124 (image 122 is blocked) uniformly fill the exit pupil 102without overlapping one another. If the beam cross section issignificantly smaller than the region 172, then the intensities of theimages 124 may be different and may change, thus causing flicker, as thebeam 106 scans across the grating 112, and the images 124 may overlapone another. Conversely, if the beam cross section is significantlylarger than the region 172, then the images 124 may be significantlysmaller than they are when the beam cross section is the same orapproximately the same size as the region 172.

FIG. 13B is a plan view of quadrants A-D of a cell region 172 of FIG.13A. The quadrants A and C have a first relief pattern that is theinverse of the second relief pattern of the quadrants B and D. “Inverse”means that the regions occupied by the surfaces 150 and 152 (light anddark regions, respectively) in one quadrant, for example quadrant A, arerespectively occupied by the surfaces 152 and 150 (dark and lightregions, respectively) of the inverse quadrant, for example quadrant D.Although specific first and second relief patterns are shown, otherrelief patterns can provide the same exit pupil 102 (FIG. 4) or asimilar exit pupil as long as the first relief pattern in quadrants Aand C is the inverse of the second relief pattern in quadrants B and D.

Referring to FIGS. 13A and 13B, although the cell regions 172 aresquare, they can have other shapes such as pentagons, hexagons or anyother shape where all the boundaries of each region 172 are shared withother regions 172, i.e., there are no “empty” spaces between the regions172.

FIG. 14 is a plan view of the interference-pattern-emanating side 148 ofthe simple binary-phase diffracting grating 112 of FIGS. 6 and 9according to an embodiment of the invention. The light and dark portionsof the grating 112 represent a relief pattern. In one embodiment, thelight portions of the grating 112 represent the raised surfaces 150(FIG. 9), and the dark portions represent the recessed surfaces 152; butin another embodiment, the dark portions represent the raised surfacesand the light portions represent the recessed surfaces. But either way,the grating 112 generates the exit-pupil images 122 and 124 of FIG. 7A.

Referring to FIG. 14, each quadrant, i.e., cell region, 174 has the samerelief pattern as the other regions 174, has dimensions d×d, andrepresents a respective pixel region of the grating 112. That is, whenthe beam 106 (FIG. 9) strikes a region 174, the grating 112 generatesthe corresponding pixels of the exit-pupil images 122 and 124 (FIG. 7A).Therefore, although omitted for clarity, the grating 112 typicallyincludes an array of regions 174, the array having the same dimensions,in pixels, as the scanned and exit-pupil images. For the reasonsdiscussed above in conjunction with FIGS. 13A and 13B, in one embodimentthe cross section of the beam 106 where it strikes the grating 112 issquare and has or approximately has the dimensions d×d, although thecross section may be circular and have or approximately have a diameterof d.

Although a specific relief pattern is shown, other relief patterns canprovide the same exit pupil 102 (FIG. 6) or a similar exit pupil.Furthermore, although the cell regions 174 are square, they can haveother shapes such as pentagons, hexagons or any other shape where allthe boundaries of each region 174 are shared with other regions 174.

FIG. 15 is a side view of an on/off diffraction grating 112 according toan embodiment of the invention discussed above in conjunction with FIGS.4-5C. When the grating 112 is “on”, it diffracts an image into multipleexit-pupil images as discussed above in conjunction with FIGS. 4-8; butwhen the diffraction grating is “off”, it effectively passes through theimage to generate the 0^(th)-order exit-pupil image 122 and only the0^(th)-order exit-pupil image.

The on/off grating 112 includes electrodes 180 a and 180 b, adiffraction-grating layer 182, and an electro-optic layer 184. Theelectrodes 180 a and 180 b connect to the respective terminals of an ACpower source (not shown), are formed from a transparent conductive filmsuch as conventional Indium Tin Oxide (ITO), and have sides 186 a and186 b and 188 a and 188 b, respectively. The diffraction-grating layer182 has an index of refraction nd and is typically similar to one of thegratings 112 discussed above in conjunction with FIGS. 4-10 and 13A-14,although the layer 182 may be similar to the gratings 112 of FIGS. 11and 12 or have other relief patterns. The electro-optic layer 184 has anindex of refraction n_(eo) that changes in response to the magnitude,direction, or both of an electric field across the layer 184, and isformed from an electro-optic material such as conventional quartz.Ideally, n_(d)=n_(e0) when there is a nonzero electric field between theelectrodes 180 a and 180 b, and n_(e0) equals the n of air when there isa zero electric field between the electrodes 180 a and 180 b. Inpractice, because n_(e0) often does not equal these ideal values andbecause the interface 189 between the layers 182 and 184 has corners190, i.e., is not planar, the relief pattern (e.g., FIGS. 13A-14) formedin the diffraction layer 182 can be altered to account for themismatched indices of refraction. Because techniques for altering therelief pattern of the layer 182 are known, they are omitted here forbrevity. Furthermore, as long as the sides 186 a, 186 b, 188 a, and 188b of the electrodes 180 a and 180 b are planar, they have little or noadverse affect on the generation of the exit pupil.

In operation, the diffraction grating 112 generates an expanded exitpupil having multiple exit-pupil images (e.g., FIG. 4), and is thus“on”, when no electric field is present across the electro-optic layer184. Specifically, when no electric field is present, n_(d)≠n_(e0), andthus the diffraction layer 182 can diffract the incident image togenerate multiple exit-pupil images.

Conversely, the diffraction grating 112 generates only the 0^(th)-orderexit pupil (e.g., FIG. 4), and thus is “off”, when a nonzero electricfield is present across the electro-optic layer 184. Specifically, whenan electric field is present, n_(d)=n_(e0) or n_(d)≈n_(e0), and thus theincident image does not “see” the relief pattern of the diffractionlayer 182 because the layers 182 and 184 effectively combine into asingle optical layer with planar sides. Therefore, the layer 182 cannotdiffract the incident image into multiple exit-pupil images.

FIG. 16 is a side view of an on/off diffraction grating 112 according toanother embodiment of the invention, where like numbers indicate likeelements with respect to FIG. 15. The grating 112 of FIG. 16 is similarto the grating 112 of FIG. 15 except that both electrodes 180 a and 180b are on one side of the diffraction layer 182. An advantage of this isthat because the electrodes 180 a and 180 b are closer together, asmaller voltage can be applied across the electrodes to generate thedesired electric field through the electro-optic layer 184. Adisadvantage, however, is that because the layers 182 and 184 are notcontiguous and the side 186 a of the electrode 180 a is not planar, therelief pattern formed in the layer 182 often should be modified toaccount for this. Because techniques for altering the relief pattern areknown, they are omitted here for brevity.

In operation, the diffraction grating 112 of FIG. 16 operates in amanner similar to that of the diffraction grating 112 of FIG. 15.

Still referring to FIGS. 15 and 16, alternative embodiments of theon/off grating 112 are contemplated. For example, one may replace theelectro-optic layer 184 with a thermo-optic layer. Lithium niobate is anexample of one material that may be used as a thermo-optic orelectro-optic layer depending upon the method of activation. Otherexamples of such materials include polled organic chromophores.Collectively, materials capable of undergoing a change in index ofrefraction may be referred to as non-linear optical materials.

A thermo-optic layer has an index of refraction that varies with thetemperature of the layer. Therefore, one can turn the grating 112 on andoff by appropriately adjusting the temperature of the thermo-opticlayer. Techniques for adjusting the temperature of the thermo-opticlayer are known, and, therefore, are not disclosed herein.

In another embodiment, one can form an on/off refractive optical elementthat can operate as an on/off exit-pupil expander by replacing thediffraction layer 182 with one or more refractive layers. For example,one can replace the diffraction layer 182 with a refractive exit-pupilexpander such as that disclosed below in conjunction with FIG. 17.

FIG. 17 is a cross-section of a refractive beam multiplier 200, whichhas a dual-microlens-array architecture and can operate as an exit-pupilexpander according to an embodiment of the invention. That is, withappropriate modifications to the ocular 114, one can replace thediffraction grating 112 with the beam multiplier 200 in the lensassemblies 110 of FIGS. 4, 6, and 8. For example, within a predeterminedcross-sectional radius in the far field, the beam multiplier 200 cantypically generate exit-pupil images (i.e., beamlets) having relativelyuniform intensities that have little or no dependence on wavelength.Therefore, one may be able to omit the obscuration plate 126.

The beam multiplier 200 includes a first light-transmissive microlensarray 202 having a focal plane 204, a lens surface 206, and a non-lenssurface 208, and includes a second light-transmissive microlens array210 having a focal plane 212, a lens surface 214 that faces the lenssurface of the first array, and a non-lens surface 216. Each of thearrays 202 and 210 has substantially the same focal length f, and thefocal planes 204 and 212 of the arrays are separated by f to define agap 218 between the arrays. The gap 218 may be filled with air or anyother light-transmissive material. Each lens 220 of the arrays 202 and210 has substantially the same diameter D.

In operation according to one embodiment of the invention, a mechanism(not shown) sweeps a light beam 106 across the non-lens surface 208 ofthe first array 202. The light beam 106 has a Gaussian intensity profileand a diameter S that is between D and 2D. The light beam 106 propagatesthrough the arrays 202 and 210 and, as it is swept, generates anexpanded exit pupil (not shown in FIG. 17) that includes multiple,substantially identical exit-pupil images in the far field adjacent tothe non-lens surface 216 of the second array 210.

In operation according to another embodiment of the invention, amechanism (not shown) generates a non-swept image beam that spanssubstantially the entire non-lens surface 208. This beam allows the beammultiplier 200 to simultaneously generate all of the pixels within eachof the exit-pupil images that compose the expanded exit pupil.

Still referring to FIG. 17, other embodiments of the beam multiplier 200are contemplated. For example, the beam 106 may have a diameter S thatis not between D and 2D and that does not have a Gaussian intensityprofile or a circular cross section. Furthermore, the beam multiplier200 can have adjustable optical characteristics like the diffractiongratings 112 of FIGS. 15 and 16. For example, the gap 218 may be filledwith a non-linear optical material. When the material has a first indexof refraction, for example the same as that of air, the multiplier 200operates as described above. But when the non-linear optical materialhas the same index of refraction as the material from which themicrolens arrays 202 and 210 are formed, the multiplier 200 passes theincident beam without generating multiple beamlets. A stimulus such asheat or an electric field can alter the index of refraction of thenon-linear optical material as described above in conjunction with FIGS.15 and 16. Alternatively, the multiplier 200 can include a structurethat alters the shapes of the lens surfaces 206 and 214 so as to alterthe refractive characteristics of the multiplier, and thus alter, e.g.,the beamlet pattern. Such a structure can be conventional, and can alterthe shapes of the lens surfaces with e.g., heat or an electric field.

FIG. 18 is a cross-section of a refractive beam multiplier 230, whichcan operate as an exit-pupil expander according to an embodiment of theinvention. The beam multiplier 230 is effectively a reflective versionof the beam multiplier 200 of FIG. 17. More specifically, the multiplier230 includes a single microlens array 232 and a reflector 234 spaced f/2from a focal plane 236 of the array.

The beam multiplier 230 operates similarly to the beam multiplier 200 ofFIG. 17 except that the multiplier 230 receives the beam 106 and formsthe expanded exit pupil on the same side of the multiplier. Furthermore,the beam multiplier 230 can have adjustable optical characteristics likethe beam multiplier 200 as discussed above in conjunction with FIG. 17.

The foregoing discussion is presented to enable a person skilled in theart to make and use the invention. Various modifications to theembodiments will be readily apparent to those skilled in the art, andthe generic principles herein may be applied to other embodiments andapplications without departing from the spirit and scope of the presentinvention as defined by the appended claims. Thus, the present inventionis not intended to be limited to the embodiments shown, but is to beaccorded the widest scope consistent with the principles and featuresdisclosed herein.

1. A beam multiplier, comprising: a beam-multiplying layer operable togenerate output beamlets of light from an input beam of light; and anoptical layer adjacent to the beam-multiplying layer and having anadjustable index of refraction.
 2. The beam multiplier of claim 1wherein the beam multiplier layer is transparent.
 3. The beam multiplierof claim 1, further comprising a reflector operable to reflect in afirst direction light passing through the beam-multiplying layer insecond direction.
 4. The beam multiplier of claim 3 wherein thereflector is integral with the beam-multiplying layer.
 5. The beammultiplier of claim 3 wherein the reflector is separate from thebeam-multiplying layer.
 6. The beam multiplier of claim 1 wherein theoptical layer is transparent.
 7. The beam multiplier of claim 1 whereinthe optical layer comprises an electro-optic layer.
 8. The beammultiplier of claim 1 wherein the optical layer comprises a thermo-opticlayer.
 9. The beam multiplier of claim 1 wherein the beam-multiplyinglayer comprises a diffractive optical element.
 10. The beam multiplierof claim 1 wherein the beam-multiplying layer comprises a refractiveoptical element.
 11. The beam multiplier of claim 1 wherein: thebeam-multiplying layer has a first index of refraction; and the opticallayer comprises an electro-optic layer having substantially the firstindex of refraction in response to a first electric field across theelectro-optic layer and having a second index of refraction in responseto a second electric field across the electro-optic layer.
 12. The beammultiplier of claim 1 wherein: the beam-multiplying layer has a firstindex of refraction; and the optical layer comprises an electro-opticlayer having substantially the first index of refraction in response toa nonzero electric field across the electro-optic layer and having asecond index of refraction in response to a substantially zero electricfield across the electro-optic layer.
 13. The beam multiplier of claim1, further comprising: wherein the beam-multiplying layer has first andsecond opposite sides; wherein the optical layer comprises anelectro-optic layer having first and second opposite sides, the firstside of the electro-optic layer adjacent to the second side of thebeam-multiplying layer; a first electrode adjacent to the first side ofthe beam-multiplying layer; and a second electrode adjacent to thesecond side of the electro-optic layer.
 14. The beam multiplier of claim1, further comprising: wherein the beam-multiplying layer has first andsecond opposite sides; wherein the optical layer comprises anelectro-optic layer having first and second opposite sides, the firstside of the electro-optic layer adjacent to the second side of thebeam-multiplying layer; a first transparent electrode adjacent to thefirst side of the beam-multiplying layer; and a second transparentelectrode adjacent to the second side of the electro-optic layer. 15.The beam multiplier of claim 1, further comprising: wherein the opticallayer comprises an electro-optic layer having first and second oppositesides; a first electrode disposed between the beam-multiplying layer andthe first side of the electro-optic layer; and a second electrodeadjacent to the second side of the electro-optic layer.
 16. The beammultiplier of claim 1, further comprising: wherein the optical layercomprises an electro-optic layer having first and second opposite sides;a first transparent electrode disposed between the beam-multiplyinglayer and the first side of the electro-optic layer; and a secondtransparent electrode adjacent to the second side of the electro-opticlayer.
 17. A display system, comprising: an image source operable togenerate an optical source image; and an exit-pupil expander operable togenerate multiple exit-pupil images from the source image in response toa first signal and to generate only a single exit-pupil image inresponse to a second signal.
 18. The display system of claim 17 wherein:the first and second signals respectively comprises first and secondelectric fields; and the exit-pupil expander comprises electrodesoperable to apply the first and second electric fields across a portionof the exit-pupil expander.
 19. The display system of claim 17 whereinthe exit-pupil expander is further operable to: generate a 0^(th)-orderexit-pupil image and at least one higher-order exit-pupil image from thesource image in response to the first signal; and generate only the0^(th)-order exit-pupil image from the source image in response to thesecond signal.
 20. The display system of claim 17, further comprising afilter operable to attenuate one of the multiple exit-pupil images inresponse to the first signal and operable to pass the single exit-pupilimage substantially unattenuated in response to the second signal.
 21. Amethod, comprising: generating beamlets from an incident light beam inresponse to a first stimulus; and passing the incident light withoutgenerating the beamlets in response to a second stimulus.
 22. The methodof claim 21 wherein: the first stimulus comprises a nonzero electricfield; and the second stimulus comprises a substantially zero electricfield.
 23. The method of claim 21 wherein: the first stimulus comprisesa first temperature; and the second stimulus comprises a secondtemperature.
 24. A method, comprising: diffracting incident light inresponse to a first stimulus; and passing the incident lightsubstantially undiffracted in response to a second stimulus.
 25. Amethod, comprising: receiving an image with an exit-pupil expander;generating multiple exit-pupil images from the received image when theexit-pupil expander is in a first state; and generating a singleexit-pupil image from the received image when the exit-pupil expander isin a second state.
 26. The method of claim 25, further comprising:causing the exit-pupil expander to be in the first state with a firststimulus; and causing the exit-pupil expander to be in the second statewith a second stimulus.
 27. The method of claim 25, further comprising:causing the exit-pupil expander to be in the first state by applying afirst electric field across a portion of the exit-pupil expander; andcausing the exit-pupil expander to be in the second state by applying asecond electric field across the portion of the exit-pupil expander. 28.The method of claim 25, further comprising: causing the exit-pupilexpander to be in the first state by taking a portion of the exit-pupilexpander to a first temperature; and causing the exit-pupil expander tobe in the second state by taking the portion of the exit-pupil expanderto a second temperature.
 29. The method of claim 25, further comprising:generating the image by sweeping an electromagnetic beam; and whereinreceiving the image comprises receiving the swept electromagnetic beamwith the exit-pupil expander.
 30. The method of claim 25 whereingenerating multiple exit-pupil images comprises diffracting the receivedimage.
 31. The method of claim 25 wherein generating multiple exit-pupilimages comprises refracting the received image.
 32. The method of claim25, further comprising: causing the exit-pupil expander to be in thefirst state by causing a portion of the exit-pupil expander to have afirst index of refraction; and causing the exit-pupil expander to be inthe second state by causing the portion of the exit-pupil expander tohave a second index of refraction.
 33. A method, comprising: diffractingincident light according to a first diffraction property in response toa first stimulus; and diffracting incident light according to a seconddiffraction property in response to a second stimulus.
 34. The method ofclaim 33 wherein: diffracting incident light according to the firstdiffraction property comprises generating outgoing light at an outgoingangle that is different than an incident angle of the incident light;and diffracting the incident light according to the second diffractionproperty comprises generating outgoing light at an outgoing angle thatis substantially the same as the incident angle of the incident light.35. The method of claim 33 wherein: the first stimulus comprises a firstvoltage; and the second stimulus comprises a second voltage.
 36. Anoptical device, comprising: a diffraction grating having a diffractioncharacteristic: and a structure operable to alter the diffractioncharacteristic of the diffraction grating.
 37. The optical device ofclaim 36 wherein the structure comprises an electrode.
 38. The opticaldevice of claim 36 wherein the structure: comprises first and secondelectrodes disposed on opposite sides of the diffraction grating: and isoperable to alter the diffraction characteristic of the diffractiongrating in response to a voltage across the first and second electrodes.39. The optical device of claim 36 wherein the structure: comprisesfirst and second electrodes disposed on a same side of the diffractiongrating: and is operable to alter the diffraction characteristic of thediffraction grating in response to a voltage across the first and secondelectrodes.
 40. The optical device of claim 36 wherein the structure:comprises first and second electrodes disposed on opposite sides of thediffraction grating: and is operable to alter the diffractioncharacteristic of the diffraction grating in response to an electricfield between the first and second electrodes.
 41. The optical device ofclaim 36 wherein the structure: comprises first and second electrodesdisposed on a same side of the diffraction grating: and is operable toalter the diffraction characteristic of the diffraction grating inresponse to an electric field between the first and second electrodes.42. A method, comprising: manipulating incident light according to afirst optical characteristic in response to a first stimulus; andmanipulating incident light according to a second optical characteristicin response to a second stimulus.
 43. The method of claim 42 wherein:manipulating incident light according to the first opticalcharacteristic comprises diffracting the incident light according to afirst diffraction characteristic; and manipulating the incident lightaccording to the second optical characteristic comprises diffracting theincident light according to a first diffraction characteristic.
 44. Themethod of claim 42 wherein: manipulating incident light according to thefirst optical characteristic comprises refracting the incident lightaccording to a first refraction characteristic; and manipulating theincident light according to the second optical characteristic comprisesrefracting the incident light according to a first refractioncharacteristic.
 45. An optical device, comprising: an optical elementhaving an optical characteristic: and a structure operable to alter theoptical characteristic of the optical element.
 46. The optical device ofclaim 45 wherein the optical element comprises a diffractive opticalelement.
 47. The optical device of claim 36 wherein the optical elementcomprises a refractive optical element.
 48. The optical device of claim36 wherein the optical element comprises an array of lenses.
 49. Theoptical device of claim 36 wherein the optical element comprises adiffraction grating.
 50. A method, comprising: tuning an opticalcharacteristic of an optical element with a stimulus; and manipulatinglight with the tuned optical element.
 51. The method of claim 50 whereintuning an optical characteristic comprises tuning an index of refractionof the optical element with a stimulus.
 52. The method of claim 50wherein tuning an optical characteristic comprises tuning a refractivecharacteristic of the optical element with a stimulus.
 53. The method ofclaim 50 wherein tuning an optical characteristic comprises tuning adiffractive characteristic of the optical element with a stimulus. 54.An optical element: having an optical characteristic that is tunable inresponse to a stimulus; and operable to manipulate light according tothe tuned optical characteristic.
 55. The optical element of claim 54wherein the optical characteristic comprises a refractivecharacteristic.
 56. The optical element of claim 54 wherein the opticalcharacteristic comprises a diffractive characteristic.
 57. The opticalelement of claim 54 wherein the optical characteristic comprises anindex of refraction.